In a manufacturing process using a lithographic projection apparatus, a pattern (e.g. in a mask) is imaged onto a substrate that is at least partially covered by a layer of radiation-sensitive material (resist). Prior to this imaging procedure, the substrate may undergo various other procedures such as priming, resist coating, and/or a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake, and/or measurement/inspection of the imaged features. This array of procedures may be used as a basis to pattern an individual layer of a device (e.g. an IC). Such a patterned layer may then undergo various processes such as etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., all of which may be intended to finish off an individual layer. If several layers are required, then the whole procedure, or a variant thereof, may be repeated for each new layer. Eventually, an array of devices will be present on the substrate (wafer). These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4.
The measurement and inspection procedure following development of the resist, referred to as “in-line” because it is carried out in the normal course of processing production wafers, serves two purposes. Firstly, it is desirable to detect any target areas where the pattern in the developed resist may be faulty. If a sufficient number of dies are faulty, the wafer can be stripped of the patterned resist and re-exposed, hopefully correctly, rather than making the fault permanent by carrying out a process procedure (e.g. an etch) with a faulty pattern. Secondly, the measurements may allow errors in the lithographic apparatus (e.g. illumination settings or exposure times) to be detected and corrected for subsequent exposures.
However, many errors in the lithographic apparatus may not be easily detected or quantified from the patterns printed in exposures. Detection of a fault does not always lead directly to its cause. Thus, a variety of “off-line” procedures for detecting and measuring errors in the lithographic apparatus are known. These may involve replacing the substrate with a measuring device or carrying out exposures of special test patterns, e.g. at a variety of different machine settings. Such off-line techniques take time, often a considerable amount, during which the apparatus may not be available for production exposures. Therefore, in-line techniques (that is, ones which can be carried out using, or at the same time as, production exposures) for detecting and measuring errors in the lithographic apparatus are preferred.
One in-line method used in device manufacturing for measurements of linewidth, pitch and critical dimension (CD) makes use of a technique known as “scatterometry”. Methods of scatterometry are described in Raymond et al., “Multiparameter Grating Metrology Using Optical Scatterometry”, J. Vac. Sci. Tech. B, Vol. 15, no. 2, pp. 361-368 (1997) and Niu et al., “Specular Spectroscopic Scatterometry in DUV Lithography”, SPIE, Vol. 3677 (1999). In scatterometry, white light is reflected by periodic structures in the developed resist and the resulting reflection spectrum at a given angle detected. The structure giving rise to the reflection spectrum is reconstructed, e.g. using Rigorous Coupled-Wave Analysis (RCWA) or by comparison to a library of patterns derived by simulation.
Pattern asymmetry is an error that can have a number of sources in the lithographic apparatus, including aberrations in the projection system, non-aberration projection lens artifacts, the reticle and the resist processing. Existing techniques for measuring pattern asymmetry are difficult, off-line and do not directly measure the effects in resist, so that it is not guaranteed that measures taken to minimize the problem will have the desired effect in the finished product.